This study will provide basic information for determining which eyes of patients with diabetic retinopathy may be successfully treated with photocoagulation. Knowing the status of the retinal capillary system would help to determine when treatment might be delivered most effectively. Using the two-point fluorophotometer, the dye dilution technique will be employed on normals and diabetic patients, in order to determine mean circulation time, segmental blood flow, and fluorescein diffusion in the optic nerve and fovea centralis. Diabetics with and without retinopathy will be evaluated before and after insulin injection or other treatment.